Flu Hits Pre-Schoolers Hard

NASHVILLE, Tenn., July 5 - Nearly one in every thousand pre-schoolers is hospitalized for seasonal influenza every year, and the rate is more than four times higher for the youngest children, according to researchers here.

And outpatient visits because of the flu are between 10 and 250 times more common than hospitalizations, depending on which age group is measured, found to Katherine Poehling, M.D., of Vanderbilt University Medical Center here, and colleagues.

What's more, fewer than 30% of those flu cases are diagnosed clinically, Dr. Poehling and colleagues reported in the July 6 issue of the New England Journal of Medicine.

The findings came from a prospective study of influenza in children, conducted in three counties in Tennessee, New York and Ohio. The initiative was part of the New Vaccine Surveillance Network (NVSN), sponsored by the CDC in Atlanta, which had a goal to determine population-based rates of laboratory-confirmed influenza and to assess the effects of recommendations regarding vaccination.

During the flu seasons of 2002-2003 and 2003-2004, vaccination was urged for all children between six and 23 months of age, and routine vaccination for those children was recommended beginning in 2004, Dr. Poehling and colleagues noted.

The recommendation for routine vaccination "was driven primarily by high hospitalization rates (but) our findings demonstrate that the outpatient burden of influenza is substantial," Dr. Poehling and colleagues concluded.

The researchers conducted a two-part study:

• In a four-year analysis beginning in 2000, they prospectively enrolled children younger than five who were admitted to the hospital for acute respiratory tract infection or fever. Nasal and throat swabs were tested for influenza virus.
• During the flu seasons of 2002-2003 and 2003-2004, they enrolled children seen in selected pediatric clinics and emergency departments. Again, the children were tested for the virus.

The analysis found that over the four-year hospitalization study, 0.9 pre-schoolers per 1,000 were admitted to the hospital with what lab tests confirmed was influenza. However, in those younger than six months the rate was 4.5 per 1,000.

The estimated burden of outpatient visits was 50 clinic visits and six emergency department visits per 1,000 children in the 2002-2003 season and 95 clinic visits and 27 emergency department visits per 1,000 children during the following flu season, the researchers found.

"Although hospitalization rates attributable to influenza are important," the researchers noted, "the average annual rates of outpatient visits attributable to influenza were approximately 10, 100, and 250 times as high as hospitalization rates for children zero to five months, six to 23 months, and 24 to 59 months of age, respectively."

Strikingly, few children who had laboratory-confirmed influenza were actually diagnosed with flu by the treating physician. In the inpatient setting, 28% of children had a flu diagnosis, while 17% of outpatients were diagnosed as having the flu.

The rates found by Dr. Poehling and colleagues are probably an under-estimate of the impact of flu on children, said Paul Glezen, M.D., of Baylor College of Medicine in Houston. Writing in an accompanying editorial, Dr. Glezen said that the study did not count children admitted after the acute phase of the infection, when the virus is no longer present, nor those in which the affected organ system was not the respiratory tract.

"It is clear," he said, "that universal immunization against influenza should be considered." The NVSN program, he added, is an "efficient platform for evaluating the effectiveness of universal immunization against influenza among preschool children."